Three-area vane type pressure energy translating device having shock absorbing valve means



Jan. 9, 1968 c. E. ADAMS 3,362,340

THREE1-AREA VANE TYPE PRESSURE ENERGY TRANSLATING DEVICE HAVING SHOCK ABSORBING VALVE MEANS Filed Dec. 9, 1965 2 Sheets-Sheet 1 INVENTOR.

BY {fa/z m Jan. 9, 1968 c. E. ADAMS 3,362,340

THREE-AREA VANE TYPE PRESSURE ENERGY TRANSLATING DEVICE HAVING SHOCK ABSORBING VALVE MEANS Filed Dec. 9, 1965 2 Sheets-Sheet 2 4a INVENTOR, 2 I BYfiaaZ 1462M, y/wpyw 3 v Arm/ W019 United States Patent THREE-AREA VANE TYPE PRESSURE ENERGY TRANSLATING DEVICE HAVING SHOCK AB- SORBING VALVE MEANS Cecil E. Adams, Columbus, Ohio, assiguor to Abex Corporation, a corporation of Delaware Filed Dec. 9, 1965, Ser. No. 512,654 Claims. (Cl. 103136) This invention is directed to improvements in pressure energy translating devices of the vane type which include hydraulic means for vane control and which are known in the art as three-area devices. More specifically, the invention relates to means for preventing damage to the third area means from shock or impact in operation.

A fluid pressure energy translating device of the vane type includes a rotor having a plurality of radially movable vanes carried in slots spaced equi-angularly around its periphery. These vanes engage the cam surface of a fixed stator or cam ring which surrounds the rotor. Inlet and outlet ports open at spaced positions into the area between the periphery of the rotor and the cam surface and are swept or traversed sequentially by the vanes as the rotor turns, whereby fluid is received at the inlet port and is transferred by the vanes to the outlet port. Depending upon whether the rotor is driven by an outside source of rotary movement or is caused to rotate by a greater pressure at the inlet port than at the outlet port, the device constitutes either a pump or a motor. For purposes of illustration, the invention is explained hereinafter primarily in relation to a hydraulic pump.

In a vane pump of the three-area type, fluid pressures act on three areas associated with each vane, and the forces resulting from these pressures cooperate to urge each vane into operative engagement with the cam surface, i.e., to maintain a dynamic seal between the vane tip and the cam surface. By utilization of the three area vane control principle, a predetermined force for urging or moving the vanes outwardly is obtained.

The fluid pressures acting on two of the areas associated with each vane are substantially equal but act in opposite directions, and the forces resulting from these pressures thus tend to counteract each other. The first area comprises a surface on the radially outer end of the vane and is subjected to pressure which urges the vane inwardly in its slot. The second vane area comprises a surface on the radially inner end of the vane which is subjected to a pressure equal but opposed to that acting on the first area, which pressure urges the vane outwardly in its slot.

A third area associated with each vane is also subjected to a fluid pressure which urges the vane outwardly. It is the pressure on this third area which, excepting centrifugal force, provides the controlling force urging the vane outwardly to effect and maintain a fluid seal between the outer end of the vane and the cam ring regardless of irregularities in the contour or arcua'ten'ess of the cam ring surface.

This invention is directed to improvements in three-area vane-type devices of the general type described in US. Patent No. 2,832,293, issued April 29, 1958, and in application Ser. No. 296,017, filed July 18, 1963, now Patent No. 3,223,044, of both of which the present applicant is one of the co-inventors.

Three area devices of the type to which this invention relates include a rotor having an internal pressurechamher and having a pressure differential operated pin or piston associated with each vane, the piston being slidable in a bore intersecting the pressure chamber and leading to the vane. Pressure in the chamber acting on the piston provides the third area force holding the vanes outwardly. Pressure is supplied into this chamber through passages Patented Jan. 9, 1968 from a high pressure zone of the pump whenever the pressure in the chamber tends to drop below the pressure in the high pressure zone.

The third area means may, for example, be of the solid piston type shown in Patent No. 2,832,293 or preferably, they may comprise a piston having an axial passage which forms a check valve with the inner end of the respective vane for regulating the flow of pressure fluid from the pressure port into the pressure chamber interconnecting the inner ends of all the pistons. Whenever the external pressure acting on a piston exceeds the pressure in the internal chamber tending to hold the piston out, fluid from the vane slot will flow into the chamber through the longitudinal passage in the piston until a substantial pressure equilibrium is reached. This pressure acts equally on all of the pistons and on the vanes to hold the vanes against the cam surface.

While three-area devices of the types described above have effected a significant improvement in operating performance, such devices have occasionally been subject to internal damage in operation. In particular, operation of such devices under some conditions has occasionally been found to result, over a period of time, in deformation or mushrooming of the inner end of the pistons and occasionally in the rupture by a piston of the opposite Wall defining the pressure conduit in the rotor. It is believed that this damage occurs where the device is subjected to sudden increases in output pressure Prior to this invention, no satisfactory method has been known for attenuating or arresting thrust of the pistons against the base of their bores. It has been, therefore, an object of this invention to provide means by which this cause of damage to the piston or pressure chamber wall structure can be arrested.

This invention is predicated upon the discovery that the otherwise adverse effect of sharp, high output pressure increases can be obviated by the provision of valve means associated with each third-area piston whereby movement of the piston toward the pressure chamber is slowed by gradual closure of the valve means in response to the external pressure surge. I have furtherfound that especially improved results are obtained if the rate of flow of fluid from the piston bore into the pressure chamber is progressively reduced as the piston approaches the structure which limits its travel in its bore.

The further objects and advantages of the present invention will be apparent from the following description, reference being made to the accompanying drawings wherein a preferred embodiment of the invention is disclosed.

In the drawings,

FIGURE 1 is an axial section of a three-area ty'pe vane pump incorporating a preferred structure in accordance with this invention;

FIGURE 2 is a transverse or radial section taken along line 2-2 of FIGURE 1; and

FIGURE 3 is an enlarged view of a single vane shown in FIGURE 2.

The three-area vane pump shown by way of illustration in the drawings includes a housing or casing formed by a body casting 1 having a generally cylindrical interior chamber, and an end cap 2 having a cylindrical boss 3 which telescopes into the end of the body and is sealed thereto by an O-ring 4.

The end wall 5 of the body opposite end cap 2 includes a bore through which the pump operating shaft 6 extends. Shaft 6 is supported for rotation in this bore by a bearing (not shown) which is secured against axial movement in the bore. Shaft 6 extends from the body 1 into end cap 2 and is carried for rotation therein by a needle type roller bearing 7 mounted within a central bore in the end cap.

Cylindrical boss 3 of the end cap is finished to form a flat inner surface or cheek plate which is clamped against a side or radial face 8 of a cam ring 9. It may be mentioned here that the cam ring itself as well as the housing and cam ring together are sometimes referred to in the art as a stator.

A fluid intake 10 extends radially into body 1 and communicates with a pair of internal annular channels 11, 12 which encircle the internal cavity within the body. These annular channels 11, 12 distribute fluid from the intake 10 to suction ports to be described.

The cam ring 9 is supported radially by an annular rib 13 formed in the body 11 between the annular channels 11, 12. The cam ring 9 encircles a rotor 14 which is connected to shaft 6 through a spline 15 that permits proper running alignment between the rotor, the flat surface of the cylindrical boss 3, and a cheek plate 16. Rotor 14 is provided with a plurality of radial vane slots 17 in each of which a vane 18 is mounted.

The cam ring 9 has a cam surface 19 that is contoured to provide a balanced construction in which there are diametrically opposite low pressure or suction zones 20, fluid transfer zones 21, high pressure or exhaust zones 22, and sealing zones 23 formed between the cam surface and the rotor 14 (see FIG. 2). In order to provide the opposed zones, the cam surface 19 is formed in part from a first pair of arcs of equal radii which extend across the fluid transfer zones 21 and, in part, by a second pair of arcs of shorter radii than the first pair of arcs which extend across the sealing zones 23. These pairs of arcs are interconnected by cam surfaces which extend across the low and high pressure zones and 22 respectively.

Cheek plate 16 is finished to provide a smooth flat radial surface on the side thereof which abuts the cam ring 9, and has a central bore 24 surrounded by a cylindrical boss 25 which extends into the bore in the wall 5 of the body 1 and is sealed thereto by an O-ring 2 6. The outer cylindrical surface of cheek plate 16 is sealed to body 1 by an O-ring 27.

The cheek plate 16 is movable axially in the body 1 and is urged toward rotor 14 by fluid pressure supplied from the high pressure zone 22 through passageways 28 and 29 to a pressure chamber 30 formed between the body and the outer face 31 of the cheek plate. The cheek plate functions in the nature of an axially movable, non-rotatable piston under the pressure supplied by the fluid in chamber 30 to maintain it in engagement with the cam ring 9.

Intake 10 communicates through annular channels 11, 12 around the cam ring 9 to suction or inlet ports spaced 180 apart. Two suction ports, one of which is shown at 50 in FIG. 2, are formed in cheek plate 15 and are fed by channel 12, and two additional suction ports (not shown) are formed in end cap 2 and are fed by channel 11. These suction ports in the end cap and cheek plate are identical in shape and are axially aligned with the suction zones 20 between rotor 14 and cam surface 19. Each suction port has a branch passage, the opening of one of which is designated at 51, whereby the suction port communicates with the inner ends 32 of vane slots 17 in the rotor 14 as well as with the inlet zones 20.

As shown in FIG. 1, the end cap 2 includes two diametrically opposed crescent-shaped exhaust or pressure ports 52, 52 which are spaced substantially 90 from the suction ports. Similarly, pressure ports 56, 56 are formed in cheek plate 16 which are axially aligned with the pressure zones 22 and with ports 52 in the end cap. Each pressure port 52. and 56 communicates with the inner ends 32 of the vane slots 17 in the rotor as the vane slots pass the ports through branch ports 54. Pressure ports 52 are connected with a fluid outlet 34 by a passageway 35 in the end cap 2.

In the direction of rotor movement (shown by the arrow in FIG. 2), the cam surface 19 progressively recedes from the periphery of the rotor 14 across the suction zones 20. In the transfer zones 21 cam surface 19 has a substantially constant radial spacing from the rotor, and across the exhaust zones 22 the cam surface progressively approaches the rotor 14 as it comes into close proximity with the periphery of the rotor 14 in the sealing zones 23. Fluid from the suction ports 50 is drawn into the fluid transport pockets defined between the successive vanes as those pockets become larger when the vanes 18 move through the suction zones 20. The fluid is positively displaced from the pockets as the volume thereof diminishes when the vanes move through the pressure zones 22, to thereby effect a pumping action.

Each vane 18 is provided with grooves 36 which are formed in its outer edge and opposite side edges. One or more channels or bores 37 are also provided in each vane which communicate between the outer groove 36 of the vane and the inner end 32 of the vane slot. The grooves 36 and channels 37 insure that the fluid pressure acting on the first area or outer end surface or tip of any given vane will be substantially balanced at all times by the pressure acting on the second area or inner end surface of that vane.

For the pump to operate at high efliciency it is necessary to maintain a continuous sealing engagement between the vane tips or outer end surface with the cam surface 19, regardless of changes in the arcuateness of the cam surface. For this purpose, one or more radial bores or piston cylinders 38 is formed in the rotor 14, extending inwardly from the inner end 32 of each vane slot 17. The bores38 communicate attheir inner ends with an annular pressure chamber 39 which may be defined in part by a groove in the rotor. Fluid can flow into and out of pressure chamber 39 only through radial bores 38. As seen in FIG. 1, the width (in the axial direction) of the opening 46 of chamber 39 into the bore 38 is preferably only a fraction of the diameter of the bore 38. The groove in the rotor which partially defines chamber 39 is closed by a sleeve 40 closely fitted and sealed in an axial recess in the rotor. This annular chamber or conduit 39 interconnects the inner ends of the radial bores 38.

A generally cylindrical pin or piston valve element 41 is received in each radial bore 38. Each piston 41 includes an axial passage 42 and is slidable in its bore 38 but is closely fitted thereto so that leakage of fluid along the external walls of the piston is negligible or minimal. The outer end of each piston 41 is conically tapered as at 43 and forms a valve with the flat inner edge surface 44 of each vane 13. The admission of fluid to the radial bores 38 is regulated by the balance of forces between the fluid pressure force acting inwardly upon the conical taper 43, tending to open the valve, and the pressure force arising from the pressure in chamber 39 together with centrifugal force, tending to close the valve. The length of the piston 41 is such as to permit it to move into and out of engagement with the flat inner edge surface 44 of the vane 18, regardless of the position of the vane in its slot. Sleeve 40 defines wall structure which may limit the inward travel of the pistons in their respective bores. As shown in FIG. 3, the vane slots 32 are preferably so positioned radially that they limit the inward travel of the vanes before a vane can press the corresponding piston 41 against sleeve 40.

As a piston moves inwardly and approaches sleeve 40, the inner end 45 of the piston forms a valve with the entrance 46 of chamber 39 to the bore 38. It will further be seen that the lower end surface of each piston is preferably chamfered, as at 47, and as the piston closely approaches sleeve 40 the inner end of the piston 41 cooperates with the sleeve 40 and bore 38 to define a chamber below the piston 41 from which fluid can escape only at a relatively restricted rate.

In operation, valve 41, 44 at the outer end of the piston functions in the manner of a check valve. When pressure at the inner end 32 of a vane slot acting upon the conical taper 43 of the piston 41 exceeds the centrifugal force on the piston and the pressure force in chamber 39, the 'piston is moved inwardly in its bore 38 and valve 41, 44 opens. Pressure fluid in the inner end 32 of vane slot 17 flows inwardly through passage 42 toward chamber 39 and restores, maintains, or increases the pressure in the chamber as necessary to balance the pressure acting to open the valve 41, 44. This action occurs when the vane slot 17 traverses pressure zone 22, for pressure in the zone 22 is. usually somewhat lower. When a vane 18 is traversing a suction zone 20, the pressure in chamber 39 exceeds the opposing pressure on end 43 of piston 41, and the piston is held against the inner edge 44 of the vane, to close valve 41, 44.

As previously suggested, it has been found that when the pump output flow is suddenly restricted, a sharp pressure rise results which is transmitted back into the pump through the outlet 34 and therethrough into the pressure zones 22 and the vane slots 17. Under these circumstances it is believed that the pressure fluid momentarily highly overbalances the pressure in chamber 39, and the valves 41, 44 are opened very rapidly. The high pressure fluid' in the vane slot tends to rush through passage 42 into chamber 39 to equalize the pressure therein. In the absence of structure in accordance with the invention, this rush of fluid apparently carries the pistons 41 radially inward causing the pistons to slam into the sleeve 40 or other abutting Wall structure, thereby damaging both the sleeve or wall and the pistons.

In a pump provided with the features of this invention, this damaging result is prevented. As the piston 41 moves toward sleeve 40 or other wall structure which defines the bottom of the piston bore 38, it forms a valve 45, 46 with the narrow entrance 46 to chamber 39. The flow of fluid into the chamber 39 is thereby restricted. The area of the orifice 46 gradually diminishes as the piston approaches the sleeve 40 until the valve 45, 46 is nearly closed. Thus, the force which causes movement of piston 41 ,is reduced and stopped before the piston impacts against the sleeve 40. Moreover, fluid trapped beneath the end surface 47 of the piston 41 tends to act as a shock absorber, thereby further slowing movement of the piston and preventing damage. Thus, a thrust arresting eflect on the downward movement of the piston 41 is established, preventing the sudden external pressure increase from damaging the piston or sleeve 40 or other wall defining structure.

' It has been found that whereas the pistons of devices without the improvement of this invention can at times become mushroomed at their inner ends and/or punch holes through the sleeve, by the means provided herein these effects are virtually eliminated.

Those skilled in the art will appreciate that this invention can readily be used with other types and embodiments of three-area vane pumps or motors, for example, such as are disclosed in the Adams et al. Patent No. 2,832,293, by providing valve means restricting and closing off the fl-ow of fluid from the bottom of the piston as the piston approaches the endwise wall structure.

The improved fluid pressure energy translating devices herein described may also be useful as motors, as previously mentioned. In operation as a motor, fluid pres-' sure to actuate the vane control means may not be initially available if the vanes are not, from the outset, in contact with the cam ring, and for this reason springs may be provided to provide a light initial force for holding the vanes outwardly.

What is claimed is:

1. In a fluid pressure energy translating device of the vane type including a rotor having radial vane slots, said rotor residing in a rotor chamber having low and high pressure ports and a peripheral wall, vanes mounted for reciprocation in said slots, each vane being provided at its outer end with sealing means making sealing contact with the peripheral wall of said rotor chamber, each vane presenting a first surface area to hydraulic pressure urging said vane radially inward in its slot, ea-ch vane presenting a second surface area to hydraulic pressure urging said vane radially outward in its slot, means for balancing the fluid pressures acting on the first and second surface areas of each vane, each vane also presenting a third surface area opposed to the first area thereof, hydraulically operated means associated with each third area for urging the respective vanes outwardly in its slot at least while the vane traverses said low pressure port, a pressure chamber interconnecting all said hydraulically operated means through individual branch conduits, and means for conducting pressure fluid from said high pressure port to said pressure chamber;

the improvement comprising,

pressure responsive valves associated with the respective hydraulically operated means and cooperating with the corresponding individual branch conduits to progressively restrict communication between the hydraulically operated means and said pressure chamber whenever the pressure in said pressure chamber becomes substantially lower than the pressure acting on the said hydraulically operated means tending to urge the vane outwardly.

2. A rotor assembly for a hydraulic fluid pressure energy translating device of the vane type, comprising,

a rotor having vanes mounted in vane slots for reciprocatory movement,

means defining a pressure chamber Within said rotor assembly for applying a pressure force to all of said vanes to urge the same outwardly in said slots,

a passage associated with each vane supplying pressure fluid into said pressure chamber, a pressure operated valve including a movable valve member in each said passage opening to permit the flow of fluid into said chamber, said movable valve member having an endwise passage therethrough through which said flow passes in moving toward said chamber when said valve is open,

an abutment positively limiting the opening movement of said movable valve member,

and shock absorbing means progressively reducing the rate of opening movement of said movable valve member as said member approaches said abutment thereby reducing the possibility of damage to said abutment and movable valve member under conditions of rapid opening of said valve.

3. A rotor assembly for a hydraulic fluid pressure energy translating device of the vane type, comprising,

a rotor having vanes mounted in vane slots for reciprocatory movement,

a piston associated with each vane, each piston being slidable in a radial bore in the rotor,

means defining a pressure chamber within said rotor assembly pressure which cooperates with said pistons to apply force to said vanes tending to urge said vanes outwardly in said slots,

passage means for supplying pressure fluid into said pressure chamber,

structure limiting inward movement of each piston in its bore,

and a valve defined by and between each said piston and the means defining said pressure chamber, said valve being progressively closed by inward movement of said piston as said piston approaches said structure.

4. A rotor assembly for a hydraulic fluid pressure energy translating device of the vane type, comprising,

a rotor having vanes mounted in vane slots for reciprocatory movement,

a piston associated with each vane, each piston being slidable in a radial bore in the rotor,

a transverse annular pressure chamber within said rotor assembly,

bores extending radially inward from the respective vane slots and intersecting said chamber at angularly spaced positions,

a pin valve element slidable in each said bore and urged away from said chamber by pressure fluid in said chamber,

each valve element including a longitudinal passage through which said pressure fluid must flow in moving toward said chamber,

a wall of said chamber positively limiting the movement of the respective valve element in response to flow through said passage,

and a valve defined by and between the end of valve element and said bore through which flow through said passage must pass to enter said chamber, movement of said valve element in response to flow through said passage progressively closing said valve as said valve element approaches said wall.

5. A rotor assembly for a hydraulic fluid pressure energy translating device of the vane type, comprising,

a rotor having vanes mounted in vane slots for reciprocatory movement,

a piston associated with each vane, each piston being slidable in a radial bore in the rotor,

a transverse annular pressure chamber within said rotor assembly,

bores extending radially inward from the respective vane slots and intersecting said chamber at angularly spaced positions,

a pin valve element slidable in each said bore and urged away from said chamber by pressure fluid in said chamber,

each valve element including a longitudinal passage through which said pressure fluid must flow in moving toward said chamber,

a wall of said chamber positively limiting the movement of the respective valve element in response to fiow through said passage,

and a valve defined by and between the end of valve element and said bore through which flow through said passage must pass to enter said chamber, movement of said valve element in response to flow through said passage progressively closing said valve as said valve element approaches said wall,

and means forming a dashpot in said bore resisting rapid movement of said piston toward said wall.

6. In a vane type fluid pressure energy translating device including a rotor having a plurality of vanes mounted in vane slots for relative radial movement, a stator presenting a cam surface for engagement by the outer ends of said vanes, and radially moveable pressure operated hollow piston valve means associated with each said vane for applying a force thereto to move the vane outwardly in engagement with said cam surface,

the improvement comprising,

thrust arresting means associated with each said piston means for progressively slowing the rate of valve-opening movement thereof in response to a large pressure diflerential at the opposite sides of said piston means.

7. A rotor assembly for a hydraulic fluid pressure energy translating device of the vane type, comprising,

a rotor having vanes mounted in vane slots for reciprocatory movement,

a radial bore extending inwardly toward the axis of said rotor from each vane slot,

a piston slidably received in each said radial bore, each said piston having a longitudinal passage,

an annular chamber in said rotor intersecting said bores, the said pistons forming valves with their respective vanes controlling the admission of pressure fluid from the vane slots through said longitudinal passages into said annular chamber,

structure positively limiting the movement of each pis- 8 ton in response to flow through the said passage therein toward said chamber,

each piston cooperating with said chamber to form a valve through which flow through said passage must pass to enter said chamber from the passage in said piston, said valve being progressively closed by movement of said piston in response to flow through said passage as said piston approaches said structure,

the said chamber having an axial dimension at its intersection with said bores which is less than the diameter of said bore.

8. A rotor assembly for a hydraulic fluid pressure energy translating device of the vane type, comprising,

a rotor having vanes mounted in vane slots for reciprocatory movement,

a radial bore extending inwardly toward the axis of said rotor from each vane slot,

a piston slidably received in each said radial bore, each said piston having a longitudinal passage,

' an annular chamber in said rotor intersecting said bores, the said pistons forming valves with their respective vanes controlling the admission of pressure fluid from the vane slots through said longitudinal passages into said annular chamber,

structure positively limiting the movement of each piston in response to flow through the said passage therein toward said chamber,

each piston cooperating with said chamber to form a valve through which flow through said passage must pass to enter said chamber from the passage in said piston, said valve being progressively closed by movement of said piston in response to flow through said passage as said piston approaches said structure,

the said chamber having an axial dimension at its intersection with said bores which is less than the diameter of said bore,

each said piston having an inner end surface which defines a valve orifice with said chamber, said surface being charnfered and cooperating with said structure to form a dashpot as said end surface approaches said structure.

9. A rotor assembly for a hydraulic fluid pressure energy translating device of the vane type, comprising,

a rotor having vanes mounted in vane slots for reciprocatory movement therein,

means forming a dual valve element associated with each vane, each said dual valve element being slidable in a radial bore in the rotor,

means defining a pressure chamber within said rotor assembly for applying a pressure force to all of said vanes tending to urge the same outwardly in said slots,

a valved passage associated with each vane for supplying pressure fluid into said pressure chamber, each said passage including a longitudinal passageway in the respective dual valve element through which passageway said pressure fluid flows in entering said chamber,

structure forming a stop positively limiting the movement of each dual valve element in response to flow through the passageway therein,

and valve means defined by and between said dual valve element and rotor through which flow must pass to enter said chamber from said passageway, said valve means being progressively closed by movement of said dual valve element in response to flow through said passageway as said dual valve element approaches said structure.

10. A rotor assembly for a hydraulic fluid pressure energy translating device of the vane type, comprising,

a rotor having vanes mounted in vane slots for reciprocatory movement therein,

a piston associated with each vane, each piston being movable in a radial bore extending inwardly in the rotor from the vane slot,

means defining a pressure chamber for applying a pressure force to all of said pistons tending to urge the same outwardly into contact with said vanes, said chamber interconnecting all said radial bores in the rotor inwardly of said pistons,

and a plurality of valve means, each said valve means being defined by and between said rotor and the respective piston, flow between each bore and said chamber passing through the respective valve means, said valve means being progressively closed by inward movement of said piston in its bore toward said chamber.

References Cited UNITED STATES PATENTS Adams et a1 103-136 Adams et al 103-136 Gardiner 103-136 Adams et al 103-136 Pettibone .a 103-136 Gardiner 103-136 Harrington 103-136 McGill 103-136 Henning et al 103-136 Adams et a1. 91-138 5 DONLEY J. STOCKING, Primary Examiner.

WILBUR J. GOODLIN, Examiner. 

1. IN A FLUID PRESSURE ENERGY TRANSLATING DEVICE OF THE VANE TYPE INCLUDING A ROTOR HAVING RADIAL VANE SLOTS, SAID ROTOR RESIDING IN A ROTOR CHAMBER HAVING LOW AND HIGH PRESSURE PORTS AND A PHERIPHERAL WALL, VANES MOUNTED FOR RECIPROCATION IN SAID SLOTS, EACH VANE BEING PROVIDED AT ITS OUTER END WITH SEALING MEANS MAKING SEALING CONTACT WITH THE PERIPHERAL WALL OF SAID ROTOR CHAMBER, EACH VANE PRESENTING A FIRST SURFACE AREA TO HYDRAULIC PRESSURE URGING SAID VANE RADIALLY INWARD IN ITS SLOT, EACH VANE PRESENTING A SECOND SURFACE AREA TO HYDRAULIC PRESSURE URGING SAID VANE RADIALLY OUTWARD IN ITS SLOT, MEANS FOR BALANCING THE FLUID PRESSURES ACTING ON THE FIRST AND SECOND SURFACE AREA OF EACH VANE, EACH VANE ALSO PRESENTING A THIRD SURFACE AREA OPPOSED TO THE FIRST AREA THEREOF, HYDRAULICALLY OPERATED MEANS ASSOCIATED WITH EACH THIRD AREA FOR URGING THE RESPECTIVE VANES OUTWARDLY IN ITS SLOT AT LEAST WHILE THE VANE TRAVERSES SAID LOW PRESSURE PORT, A PRESSURE CHAMBER INTERCONNECTING ALL SAID HYDRAULICALLY OPERATED MEANS THROUGH INDIVIDUAL BRANCH CONDUITS, AND MEANS FOR CONDUCTING PRESSURE FLUID FROM SAID HIGH PRESSURE PORT TO SAID PRESSURE CHAMBER; THE IMPROVEMENT COMPRISING, PRESSURE RESPONSIVE VALVES ASSOCIATED WITH THE RESPECTIVE HYDRAULICALLY OPERATED MEANS AND COOPERATING WITH THE CORRESPONDING INDIVIDUAL BRANCH CONDUITS TO PROGRESSIVELY RESTRICT COMMUNICATION BETWEEN THE HYDRAULICALLY OPERATED MEANS IN SAID PRESSURE CHAMBER WHENEVER THE PRESSURE IN SAID PRESSURE CHAMBER BECOMES SUBSTANTIALLY LOWER THAN THE PRESSURE ACTING ON THE SAID HYDRAULICALLY OPERATED MEANS TENDING TO URGE THE VANE OUTWARDLY. 